Is it practical to precamber reinforced concrete slabs?

[Tomfh]

I've heard some engineers say it's ok - you specify precamber into the formwork and then maintain constant slab thickness.

I've heard other engineers say forget it, it's impractical and you don't have enough control.

I tend to agree with the latter. What do you think?

For those who specify it, how do you make sure it happens?

 

[slickdeals]

We have specified it in the past on some longer spans. I am not sure we have done anything specific to verify that it has been provided in the slab, although we have ensured in communications with the contractor that they are aware of the requirement and we have a paper trail. End of the day, it's the contractor's responsibility to build per the contract documents.

 

[MotorCity]

It depends. If you specify a large enough camber, say an inch or more, I think one can reasonably expect to achieve the camber or relatively close to it. If you specify a quarter inch of camber, I think you are fooling yourself. It wont happen.

 

[JAE]

ACI 318 doesn't mention cambering much - there is some ACI discussion in 347 (formwork).
Here's a snippet of one of the sections:

(love how architects are referred to....grrrr)

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[SlideRuleEra]

Cast in place highway bridge decks are required to be precambered:

SCDOT Standard Specifications

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[rapt]

Precamber can be used to reduce total deflection, basically the deflected shape that you see. Normally it is limited to less than the expected short term SW deflection.

It has no effect on the amount of movement which is what effects anything on top of the slab, such as masonry partitions or brittle finishes, glass curtain walls etc. Also has no effect on vibrations.

I would not think the objective is to maintain constant slab thickness, except possibly in a precast structure where the shoring is very widely spaced. In my experience it is used for deflection control.

 

[BAretired]

I don't believe that two way camber is practical, but it seems to me that one way camber could be beneficial and more easily achieved with fly forms.

BA

 

[Tomfh]

Rapt, to clarify yes I was referring to deflection control. Eg precambering the short term dead load to reduce long term overall deflection.